This invention relates to a multistep process for evaporating polymer solutions of thermoplastic polymers and for removing volatile constituents from thermoplastic polymers by indirect heat exchange. The invention relates in particular to a process for evaporating volatile constituents out of polymer solutions using shell-and-tube heat exchangers, coiled-tube evaporators and extrusion evaporators having a special geometry.
The removal of volatile constituents from a polymer solution is one of the final process steps in the preparation of many polymers. The volatile constituents to be removed may be either solvents and/or unpolymerised monomers. Depending upon the degree of viscosity of the polymer solution, several variants of the procedure for removing the volatile constituents from the polymer solution are known, in each of which the polymer solution is heated to above the vaporisation temperature of the volatile constituents. The known apparatus includes, for example, film evaporators, extruders and equipment with indirect heat exchange.
It is crucial that the polymer is not thermally damaged during the heating of the polymer solution. Thermal damage gives rise to unwanted changes in colour or to the presence of specks in the polymer.
Thus EP-A 150 225 describes an apparatus equipped with two serially arranged heat exchangers. The heat exchangers have horizontally arranged rectangular ducts, wherein the polymer solution is degassed. This apparatus is used chiefly for the two-step heating or cooling of viscous polymer solutions during the reaction, but is relatively expensive to manufacture and in operation.
EP-B 226 204 discloses a process and a heat exchanger for removing volatile constituents from a polymer solution containing at least 25 wt. % polymer. The polymer solution is heated in a zone consisting of a multiplicity of ducts in which indirect heat exchange takes place. The ducts have a substantially consistent ratio of surface area to volume, in the range of 0.158 to 1.97 mm-1, a height of 1.27 to 12.7 mm, a width of 2.54 to 10.16 cm and a length of 1.27 to 30.48 cm. The polymer solution is heated in the ducts, at a pressure of 2 to 200 bar, to a temperature above the vaporisation temperature. of the volatile constituents but below the boiling temperature of the polymer. The residence time of the polymer solution in the ducts is 5 to 120 seconds. After the heating, the solution is transferred into a chamber in which at least 25% of the volatile constituents are vaporised out of the solution. The procedure decreases the thermal damage because the time for which the polymer is exposed to elevated temperatures has been decreased. But the process has the disadvantage that a complete removal of the solvent in one step is not possible. Furthermore, deposits of polymer form on the outer surface of the heat-exchanger and in the course of time these carbonise and occasionally flake off, so that the polymer freed from the solvent is contaminated.
EP-B 352 727 discloses a process for removing volatile constituents from polymer solutions by heating the polymer solution, in a multiplicity of parallel ducts, to a temperature above the vaporization temperature of the volatile constituents. The ratio of the heat exchanging surface area to the volumetric flow rate of the product is  greater than 80 m2/m3/h. The flow rate in the ducts is  less than 0.5 mm/s and the residence time of the polymer solution in the ducts is 120 to 200 seconds. This process also has the disadvantage that a complete removal of the solvent in one step is not possible. Furthermore, deposits of polymer form on the outer surface of the heat exchanger and in the course of time these carbonise and occasionally flake off, so that the polymer freed from the solvent is contaminated.
The published patent application EP-A 451 602 makes known a process for concentrating polymer solutions, wherein the preheated solution is slowed down in a coiled flow tube and released, and the concentrated solution is further concentrated in a second drying apparatus with self-cleaning elements which is connected immediately downstream. The process has the disadvantage that the final concentration of solvent in the polymer is still comparatively high and that the residence time in the downstream drying apparatus is too long. Particles of polymer, which lower the quality of the product, may collect on the inner walls of the coiled tube.
The object of the present invention is to provide a process for removing volatile constituents from a polymer solution which does not have the disadvantages of the known prior art processes.
This object was achieved by the development and provision of the process according to the invention described in more detail below.